Hydroxy compounds
A-Level Chemistry Topic 16 8:20 English narration · English + 中文 subtitles burned in
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Squeeze hand sanitiser onto your hands. It feels cold, and then it is gone.
把免洗洗手液挤在手上，先是一阵凉，然后就不见了。
That is ethanol — the alcohol that kills the microbes.
那就是乙醇——杀死微生物的那种醇。
Ethanol is only a short carbon chain with one small group on the end: an oxygen joined to a hydrogen.
乙醇不过是一条很短的碳链，末端多了一个小小的基团：一个氧连着一个氢。
Add that one group to a hydrocarbon and almost everything changes — how it dissolves, how it boils, and what it reacts with.
把这个基团接到烃上，几乎一切都变了——它怎样溶解、在多少度沸腾，以及它会和什么反应。
Welcome to hydroxy compounds.
欢迎来到羟基化合物。
We will make alcohols, sort them into three classes, oxidise them, and meet every test the examiner can ask you for.
我们会制取醇、把它们分成三类、把它们氧化， 并且认识考官可能考到的每一个检验方法。
Let's begin.
让我们开始吧。
Start by making them.
先从制取开始。
Six routes, and each one is really a set of reagents to remember.
考纲给出六条路线，每一条其实就是一组必须记住的试剂。
Electrophilic addition of steam to an alkene over a phosphoric acid catalyst.
烯烃的亲电加成：水蒸气加磷酸催化剂。
Treat an alkene with cold, dilute, acidified manganate seven and you get a diol — two hydroxyl groups.
让烯烃与冷的、稀的、酸化的高锰酸钾反应， 得到二醇——两个羟基。
Substitution of a halogenoalkane with aqueous sodium hydroxide and heat.
卤代烷的取代：氢氧化钠水溶液加热。
Reduction of an aldehyde or a ketone with sodium borohydride.
醛或酮的还原：硼氢化钠。
Reduction of a carboxylic acid, which needs lithium aluminium hydride.
羧酸的还原需要氢化铝锂。
Or hydrolysis of an ester with dilute acid or alkali, and heat — the reverse of how the ester was made.
或者酯的水解：稀酸或稀碱加热—— 这其实就是生成这个酯的逆过程。
Look closely at that first route.
仔细看第一条路线。
The double bond is an electron-rich pair sticking out, so it grabs a slightly positive hydrogen.
双键是一对富电子、向外突出的电子， 所以它会抓住一个带微正电的氢。
The picture uses hydrogen bromide, but steam behaves the same way.
画面用的是溴化氢，但水蒸气的表现是一样的。
The hydrogen joins the carbon that already has more of them, because that leaves the more stable ion.
这个氢会接到本来氢就更多的那个碳上，因为这样留下的正离子更稳定。
So propene and steam give propan-2-ol, not propan-1-ol.
所以丙烯与水蒸气反应，得到的是丙-2-醇，而不是丙-1-醇。
Now the single most useful idea in this topic.
现在是这一章最有用的一个想法。
Look only at the carbon that carries the hydroxyl group, and count the other carbons joined to it.
只看连着羟基的那个碳，数一数它还连着几个碳。
One carbon attached makes it primary. Two make it secondary. Three make it tertiary.
连一个碳就是伯醇，连两个是仲醇，连三个是叔醇。
You can also count hydrogens on that same carbon: primary has two, secondary has one, tertiary has none — and that missing hydrogen decides everything that follows.
你也可以数同一个碳上的氢： 伯醇有两个，仲醇有一个，叔醇一个也没有——正是这个少掉的氢决定了后面的一切。
Oxidise them.
现在来氧化它们。
The reagent is acidified potassium dichromate, or acidified manganate seven, and in an equation we write the oxygen it supplies as a capital O inside square brackets.
试剂是酸化的重铬酸钾，或者酸化的高锰酸钾； 写方程式时，我们把它提供的氧写成方括号里的一个大写 O。
A primary alcohol is oxidised to an aldehyde, and then, if you keep going, on to a carboxylic acid.
伯醇被氧化成醛，如果继续氧化，就会变成羧酸。
A secondary alcohol is oxidised to a ketone, and it stops there.
仲醇被氧化成酮，到此为止。
A tertiary alcohol is not oxidised at all, because there is no hydrogen on that carbon to remove.
叔醇根本不被氧化，因为那个碳上没有氢可以被夺走。
So one primary alcohol gives two possible products, and the apparatus decides which.
所以同一个伯醇可以给出两种产物，而决定的是仪器。
Distillation lets the aldehyde escape the moment it forms: it boils lower than the alcohol, so it leaves the flask before it can be oxidised again.
蒸馏让醛一生成就跑掉： 醛的沸点比醇低，所以它在被再次氧化之前就离开了烧瓶。
Reflux does the opposite. A vertical condenser sends every drop of vapour back into the boiling mixture, so the oxidation is driven all the way to the acid.
回流恰好相反： 竖直的冷凝管把每一滴蒸气都送回沸腾的混合物里，于是氧化被一路推到羧酸。
Nothing can escape, so the reaction cannot stop halfway.
什么都跑不掉，反应也就不会停在半路。
And there is a colour to watch.
还有一个颜色要盯住。
Acidified potassium dichromate is orange.
酸化的重铬酸钾是橙色的。
When it oxidises a primary or a secondary alcohol, the chromium is reduced and the solution turns green.
当它氧化伯醇或仲醇时， 铬被还原，溶液变成绿色。
With a tertiary alcohol nothing happens, so it stays orange.
遇到叔醇则什么也不发生，所以它仍然是橙色。
But be careful: that colour cannot separate a primary alcohol from a secondary one. Both turn it green.
但要小心：这个颜色分不出伯醇和仲醇，因为两者都会让它变绿。
Let's use all of that.
我们把这些用起来。
Three unlabelled bottles hold butan-1-ol, butan-2-ol and 2-methylpropan-2-ol.
三个没有标签的瓶子里装着丁-1-醇、丁-2-醇和2-甲基丙-2-醇。
How does warming each with acidified dichromate tell them apart?
把每一个都与酸化的重铬酸钾加热，怎样把它们区分开？
Pause the video and try it.
先暂停，自己试一试。
Ready?
好了吗？
Butan-1-ol is primary, so the dichromate turns green, and distilling gives butanal, an aldehyde.
丁-1-醇是伯醇，所以重铬酸钾变绿，蒸馏可以收集到丁醛，一种醛。
Butan-2-ol is secondary, so it also turns green, but the product is butanone, a ketone.
丁-2-醇是仲醇，也会变绿，但产物是丁酮，一种酮。
2-methylpropan-2-ol is tertiary, so the solution stays orange.
2-甲基丙-2-醇是叔醇， 所以溶液仍是橙色。
To split the first two, test the products with Tollens' reagent: the aldehyde gives a silver mirror, the ketone gives nothing.
要区分前两个，就用托伦试剂检验产物：醛会生成银镜，酮不会。
Now the equation examiners actually ask for.
再看考官真正会考的那个方程式。
Propan-1-ol, refluxed with acidified dichromate, gives propanoic acid.
丙-1-醇与酸化的重铬酸钾回流，生成丙酸。
Write the oxidising agent as oxygen in square brackets, then balance.
把氧化剂写成方括号里的氧，然后配平。
Going to the aldehyde needs one oxygen and makes one water. Going all the way to the acid needs two, and still makes one water.
氧化到醛需要一个氧，生成一分子水； 一路氧化到酸需要两个氧，同样生成一分子水。
Count the hydrogens on both sides every single time.
每一次都要数一数两边的氢。
Alcohols do more than oxidise.
醇不只会被氧化。
They burn: complete combustion gives carbon dioxide and water.
它们能燃烧：完全燃烧生成二氧化碳和水。
They react with sodium metal, fizzing gently to give hydrogen and a sodium alkoxide — the same reaction water does, only slower.
它们能与金属钠反应，缓缓冒泡，生成氢气和一种醇钠——和水的反应一样，只是慢一些。
And they can be turned into halogenoalkanes, using a hydrogen halide, phosphorus pentachloride, phosphorus trichloride and heat, or thionyl chloride.
它们还能变回卤代烷，用卤化氢、五氯化磷、三氯化磷加热，或者氯化亚砜。
Phosphorus pentachloride also gives steamy fumes of hydrogen chloride, itself a quick test for a hydroxyl group.
其中五氯化磷还会产生氯化氢的白雾，这本身就是检验羟基的方法。
Take the water back out and you get an alkene.
再把水脱掉，你就得到烯烃。
This is dehydration, and it is an elimination: the hydroxyl group leaves one carbon, a hydrogen leaves the carbon next door, and a double bond forms between them.
这叫脱水，属于消去反应：羟基从一个碳上离开， 一个氢从旁边的碳上离开，两者之间形成双键。
Learn these conditions exactly, because the mark scheme wants them: hot aluminium oxide, or concentrated sulfuric acid, or concentrated phosphoric acid — with heat.
这些条件要一字不差地记住， 因为评分标准就要它们：热的氧化铝，或浓硫酸，或浓磷酸——并且加热。
That is also step one of a classic two-step synthesis: alcohol to alkene, then cold dilute manganate seven to make a diol.
这也是经典两步合成的第一步：醇变成烯烃，再用冷的稀高锰酸钾生成二醇。
Alcohols also make esters by ester formation — the molecules that give fruit their smell.
醇还能通过酯的生成反应制酯——让水果散发香味的那类分子。
Warm an alcohol with a carboxylic acid and a little concentrated sulfuric acid as catalyst, and you get an ester plus water.
把醇与羧酸加热， 再加少量浓硫酸作催化剂，就得到酯和水。
The reaction is reversible, so the yield is never complete.
这个反应是可逆的，所以产率永远不会完全。
Naming trips people up: the alcohol part comes first, the acid part second, ending in oate.
命名最容易出错：醇的部分写在前面，酸的部分写在后面，以酸酯结尾。
Ethanol plus ethanoic acid gives ethyl ethanoate.
乙醇加乙酸，得到乙酸乙酯。
One more test, and a very precise one.
还有一个检验，而且非常精确。
Warm your compound with iodine in alkaline solution.
把你的化合物与碘在碱性溶液中加热。
A pale yellow precipitate means the molecule contains one exact fragment: a carbon carrying a hydroxyl group, one hydrogen, and a methyl group.
出现淡黄色沉淀，就说明分子里含有一个特定的片段：一个碳上连着羟基、一个氢和一个甲基。
The yellow solid is tri-iodomethane, and the other product is a carboxylate ion with one carbon fewer.
那黄色固体是三碘甲烷，另一个产物是少一个碳的羧酸根离子。
So ethanol and propan-2-ol are positive; methanol and propan-1-ol are not, because neither has a methyl group on the carbon carrying the hydroxyl.
所以乙醇和丙-2-醇是阳性的；甲醇和丙-1-醇不是， 因为它们连着羟基的那个碳上都没有甲基。
Finally: the acidity of alcohols.
最后：醇的酸性。
Are they acids?
醇是酸吗？
Only very weakly.
只是非常弱的酸。
The hydroxyl hydrogen can come away, leaving a negative alkoxide ion. That is why sodium works at all. But an alcohol is a weaker acid than water.
羟基上的氢可以离开，留下一个带负电的烷氧负离子—— 这正是钠能反应的原因。
The reason is the alkyl group: it pushes electron density onto the oxygen, so the charge on the alkoxide is less stable.
但醇是比水更弱的酸。 原因在于烷基：它把电子密度推向氧， 于是烷氧负离子上的负电荷更不稳定。
A less stable ion means the alcohol holds its hydrogen more tightly.
离子越不稳定，醇就把氢抓得越紧。
You can watch it: sodium fizzes more slowly in ethanol than in water, though both give off hydrogen.
这一点你可以直接看到：钠在乙醇中冒泡比在水中慢，尽管两个反应都放出氢气。
Three marks students throw away.
三个学生常丢的分。
First, classify the alcohol before you write anything — primary, secondary or tertiary decides every product in this topic.
第一，动笔之前先判断醇的类别——伯、仲还是叔， 它决定了这一章里的每一个产物。
Second, in an oxidation equation use oxygen in square brackets and balance it: one for the aldehyde, two for the acid.
第二，写氧化方程式时要用方括号里的氧并配平： 到醛用一个，到酸用两个。
Name the apparatus too — distillation for the aldehyde, reflux for the acid.
仪器也要写出来——蒸馏得到醛，回流得到羧酸。
Third, give the conditions in full, and the observation as orange to green, never just a colour change.
第三，条件要写全，现象要写橙色变绿色，而不是只说颜色变了。